1. Field of the Invention
This invention relates to a sensor, especially an electrode structure in a field effect transistor type sensor.
2. Description of the Prior Art
A field effect transistor (hereinafter, referred to as FET) type sensor, which compries an FET device incorporated with a sensitive means exhibiting an electric variation of electrostatic capacity, electric conductivity, electrostatic potential, etc., due to a physical or chemical interaction with the physical quantity to be detected, detects the said physical quantity as a variation of the gate operation of the said FET device. Taking advantage of the high input impedance and the amplifying function of the FET device, such an FET type sensor can exhibit a high output, even though its size is extremely small, and thus is advantageous in actual use. For the incorporation of an FET device with a sensitive means, electrodes are required to be formed on a substrate containing silicon oxide or silicon nitride. Thus, for materials for the electrodes must tightly bind to silicon oxide and/or silicon nitride.
On the other hand, depending upon the kind of sensitive means required, the sensitive means is formed on the electrodes in an atmosphere at a high temperature and/or a high humidity. Thus, the electrode materials must be stable to such an atmosphere. Moreover, since the FET type sensor, which is designed to be used as a gas sensor, a moisture sensor, an ion sensor, a biological sensor, etc., is exposed to an atmosphere due to the necessity of an interaction between the atmosphere and the sensitive means, the electrode materials must be also stable to the atmosphere.
Although aluminium, etc., which is used as an under-gate electrode in the FET device, is excellent in binding to the gate insulating film such as silicon oxide, silicon nitride, etc. it is readily oxidized or corroded in a high temperature and/or high humidity atmosphere or in an external atmosphere. This, aluminium, etc., is not suitable for an electrode material to be used for sensors. On the other hand, noble metals, such as platinum, gold, silver, etc., are stable in a high temperature and high humidity atmosphere and in an external atmosphere, but are inferior in binding to silicon oxide, silicon nitride, etc., and thus are not suitable for electrode materials for FTt type sensors.